The present invention relates to a method of evaluating a creep life consumption rate of an equipment member, which uses a maximum value of occupation ratio of creep voids at a crystal grain boundary and considers a correspondence of an actual life to the life on a surface, in order to evaluate the creep life consumption rate and a future lifetime of a member having been subjected to creep deterioration.
In order to steadily operate a thermal electric power plant for a long period of time, it is necessary to accurately know the life of equipment. For 80 percent or more of commercial thermal power generation units now located in Japan, the accumulative operation time exceeds 100,000 hours, and for about 20 percent of them, the accumulative operation time exceeds 200,000 hours (Reference 1: Keiichi Iwamoto, Thermal Nuclear Power Generation, 48-8 (1997), 14). Therefore, it is important from the viewpoint of preventive maintenance as well as cost reduction that diagnosis of the future lifetime of a boiler etc. of aged thermal power generation unit be performed at the time of regular inspection, the state of equipment be accurately grasped, and proper repairs be made.
In weld portions of a high-temperature member of thermal power generation boiler, minute pores called creep voids (hereinafter referred to as voids) are developed in the metallographic structure due to the progress of creep damage caused by the long-term use. These voids grow and connect with each other to form a microcrack of one grain boundary length. Subsequently, the formed microcracks propagate and coalesce repeatedly, which leads to breakage of the whole of the member.
At present, the creep life is evaluated by non-destructive inspection, which is made by grinding and corroding the material surface. The non-destructive inspection method includes the parameter method using a replica (Reference 4:
The Iron and Steel Institute of Japan, Creep and creep fatigue damage manual using replica method xe2x80x9cResult reports of high-temperature strength WG, Reliability evaluation technical departmentxe2x80x9d (separate-volume manual), (1991), 1) such as the A parameter method, structure comparison method, void area ratio method (Reference 2: for example, Isamu Nonaka, Keisuke Sonoya, Masashi Nakadai, Hiroshi Yoneyama, and Masaki Kitagawa, Ishikawajima-Harima Technical Report, 32-5(1992), 313), void surface density method, and grain boundary damage method (Reference 3: Kenji Kikuchi and Yoshiyuki Kaji, Material, 44-505(1995), 1244), and the ultrasonic noise energy method and the ultrasonic spectroscopy method (Reference 5: edited by The Japan Society of Mechanical Engineers, Technology for evaluating future lifetime of power plant and structure, (1992), 89, published by Gihodo Shuppan).
In the evaluation method in which attention is given to the voids at the grain boundary, it has been clarified that the void occupation ratio on the grain boundary line (Reference 6: Tsuneyuki Ejima, Syu, Ryuichi Ohtani, Takayuki Kitamura, and Naoya Tada, 32nd High-temperature Strength Symposium Procedings, (1994), 94) has a clear physical meaning as a damage parameter (Reference 7: Naoya Tada, Satoshi Fukuda, Takayuki Kitamura, and Ryuichi Ohtani, Material 46-1, (1997), 39;
Reference 8: Naoya Tada, Takayuki Kitamura, and Ryuichi Ohtani, Material 45-1, (1996), 110).
That is, the evaluation of creep life consumption rate and future lifetime has so far been made by the void area ratio in the unit area range in a predetermined region (void area ratio method), or by the ratio of grain boundary at which voids are developed to the number of intersections of a straight line and a grain boundary (A parameter method), the straight line being drawn in the direction of the principal stress in a predetermined region.
However, the above-described conventional methods have problems with the life evaluation system of equipment member; for example, the evaluation is too conservative when the creep life consumption rate obtained by the conventional method is compared with the creep life consumption rate obtained by a destructive test of the same portion, and the mechanism of actual creep rupture is not evaluated directly.
Also, in some cases, the stress direction is considered. Since the member of equipment used actually (hereinafter referred to as actual equipment) has a multiaxial stress field, the conventional methods are impractical. Also, since many evaluation points must be set, much time is required for the quantification of void development state and the estimation of creep life.
The present invention has been achieved in consideration of the above circumstances, and an object thereof is to solve the above problems and to provide a method of estimating the creep life consumption rate with high accuracy.
To solve the above problems, the present invention is characterized in that a maximum creep void/grain boundary ratio (MB) of actual equipment member is determined by using a master curve corrected considering the size etc. of equipment member from the result obtained by a rupture test piece of actual equipment member size and a test piece in which the actual equipment member is simulated, by which the creep life consumption rate of the whole of equipment member can be estimated with high accuracy.
The maximum creep void/grain boundary ratio (MB) is expressed by Equation 1.
Maximum creep void/grain boundary ratio (MB)       Maximum    ⁢          xe2x80x83        ⁢    creep    ⁢          xe2x80x83        ⁢    void    ⁢          /        ⁢    grain    ⁢          xe2x80x83        ⁢    boundary    ⁢          xe2x80x83        ⁢    ratio    ⁢          xe2x80x83        ⁢          (      MB      )        =            MAX              α        =        1            m        [                            ∑                      i            =            1                    n                ⁢                  l          ai                            L        a              ]  
In Equation 1, Lxcex1 is the total length of one grain boundary at which a creep void exists, n is the number of creep voids at a grain boundary having the total length of Lxcex1 of one grain boundary at which the creep voids exist, m is the number of grain boundaries at which a creep void exists, and 1xcex1 is a void length which is the length of the intersection of a grain boundary and a void along the grain boundary. The maximum creep void/grain boundary ratio (MB) will be referred to more simply as xe2x80x9cmaximum void/grain boundary ratio (MB)xe2x80x9d in some cases.
Also, the present invention is characterized in that in order to obtain the correspondence of the creep life of equipment member to the creep life consumption rate of member surface, an equipment member creep life consumption rate a at the time when the maximum creep void/grain boundary ratio (MB)=1 is determined by an acceleration creep test or a creep analysis, and the equipment member creep life consumption rate xcex1 is used.